(>00 MR WILLIAM SWAN ON THE GRADUAL PRODUCTION OF 



sion. If this proportionality between the duration and the apparent brightness of a 

 light be supposed to extend beyond the limits of the experiments, so as to include 

 nearly the whole time required for the production of a complete impression, it 

 would obviously follow that light requires about ith of a second to produce its 

 full effect on the eye ; and this conclusion, it will be observed, agrees with the 

 result of du-ect observation. 



The following inferences may be derived from the laws of vision which 

 have now been investigated. 



1. Personal Equation in Astronomy. 



It is well known, that different observers assign different times to the occur- 

 rence of the same astronomical phenomenon ; as, for example, the passage of a 

 star across the meridian wire of a transit instrument. The correction to be ap- 

 plied to reduce the observations, or personal equation, as it is termed, frequently 

 amounts to a considerable quantity. 



It might at first be supposed, that this discrepancy between the results of 

 different observers, may be occasioned by light acting on their eyes with unequal 

 degrees of rapidity. But on considering the manner of observing the transit of a 

 stai', it will appear that this explanation is insufiBcient. In order to estimate the 

 exact time at which the star passes one of the wires, the observer endeavours to 

 recollect the position of the star on one side of the wire, at the instant when he 

 heard the clock beat. At the next beat of the clock, the star has passed to the 

 other side of the wire ; and the observer then, by the eye, subdivides into equal 

 parts the space between the positions occupied by the star at the successive beats 

 of the clock, and estimates how many of those parts are contained in the interval 

 between the wire and the first position of the star. The magnitude of that interval 

 estimated in this manner, determines the fraction of a second to be added to the 

 time given by the clock. If, then, the discrepancy between the results of different 

 observers is to be regarded as a phenomenon of vision, it must depend upon some 

 cause which displaces the image of the star, and thereby alters its apparent dis- 

 tance from the wire. Now as the star passes across the field of the telescope, its 

 light falls successively upon different parts of the retina, illuminating each portion 

 for a very small space of time ; and if light acted on the eye of one observer more 

 rapidly than on that of another, the obvious consequence would be that the 

 image of the star would appear brighter to the person whose retina was most 

 quickly impressed by light. The only other effect which the gradual action of 

 light on the eye seems capable of producing, is to render the advancing edge 

 of the image of the star so faint, owing to the extremely short time during 

 which its light acts on the eye, as to become imperceptible when contrasted with 

 the succeeding parts of the image : for these fall upon points of the retina over 



